1. Field of the Invention
The present invention relates to starch-based corrugating adhesive compositions. In particular, the invention has to do with a dry blend composition which easily can be rehydrated with water to make a corrugating adhesive. The adhesive is made without boron compounds and with minimal or no added alkali.
2. The Related Art
There is ever increasing customer pressure to reduce or eliminate the chemicals normally used in corrugating adhesive formulations. This market trend is being driven by environmental discharge and safety concerns. There is also a need for corrugating adhesives which are easy to use and which are stable over long periods of time. Commercially acceptable corrugating adhesives having these characteristics have not heretofore been described.
In the corrugating process, adhesive is commonly applied to the tips of the flutes of a corrugated medium. Then a noncorrugated flat paper liner is applied against the adhesive coated flutes as they pass between a corrugating roll and a pressure roll. The resulting product has the corrugating medium on one side and a flat liner on the other side and is called a single-face portion. The single-face portion may be used "as is" (called a "single face" board) or adhesive may be applied to the flute tips of the single-face portion and a second flat sheet can be applied in the same manner as the first in what is called a "double-face" or a "double-back" operation. The second liner sheet is treated with heat and reduced pressure (relative to the pressure used to make a single-face portion) immediately following contact with the adhesive.
Starch-based adhesives which can be of the carrier, no-carrier and carrier-no-carrier type are commonly used in processes for manufacturing corrugated paper board. In carrier type adhesives, a portion of the starch (or dextrin) forms a carrier, often known as the gelatinized phase, which suspends the balance of the starch which is in an ungelatinized state. Under conditions of heat and pressure, the ungelatinized starch is rapidly hydrated and gelatinized to increase quickly the viscosity and adhesivity of the adhesive composition. In no-carrier type adhesives, all of the starch is slightly cooked or swollen with heat and caustic soda for viscosity. Finally, carrier-no-carrier type adhesives have a portion of the starch which forms a carrier and is responsible for about one half of the viscosity and the remaining viscosity is obtained by slightly swelling the uncooked starch.
A water soluble adhesive composition is described in U.S. Pat. No. 2,716,613. The adhesive is comprised of a lignocellulose which is hydrolyzed under elevated temperature and pressure in the presence of moisture. A water soluble reaction product is recovered and concentrated to dryness. Rehydration is accomplished by forming a slurry with methanol and refluxing while passing ammonia therethrough. This composition also can be filtered and dried.
A hemicellulose additive for paper coating compositions is described in U.S. Pat. No. 2,772,981. The hemicellulose is described as a composition that can completely replace the adhesive component in paper coatings, but preferably it is added with starch, dextrin or the like. The patent does not describe a dry rehydratable composition.
Corrugating adhesives having a solubilized fiber component are described by Fitt in U.S. Pat. No. 5,358,559 and by Giesfeldt et al. in U.S. Pat. No. 5,503,668. These are wet adhesives employing in situ solubilization processes.
Unlike the present invention, the foregoing references do not describe dry, rehydratable adhesive compositions having commercially acceptable characteristics for use in the corrugating industry.
In the present specification and claims, all parts and percentages are by weight/weight (w/w) unless otherwise specified.